1. Either Mitt Romney’s entry into the race is completely ridiculous and sad, or it is not at all ridiculous, forcing the rest of the field to respond to the challenge of his entry.
2. Since there is a real question about this matter, Mitt Romney cannot himself be the frontrunner.
3. If his entry is not ridiculous, then anyone who does not want Romney to be President now has to very quickly rally behind someone who clearly can beat him and unite the party, so as to prevent a recurrence of the 2012 primaries.
4. Jeb Bush is the only candidate currently running who clearly fits that bill: capable of raising a lot of money very quickly, possessed of near-universal name-recognition, and acceptable to both the establishment and a significant segment of the grassroots (specifically, religious conservatives).
5. Hence, if Romney’s entry is not ridiculous, Jeb Bush is the front-runner.
6. If, on the other hand, Mitt Romney’s entry into the race is ridiculous, then his main impact on the race will be on the early phase of the “invisible primary.” His entry will make it difficult for Romney donors and influential backers to say yes to somebody else, even if they take their time saying yes to him because of his ridiculousness.
7. Jeb Bush, however, has (as noted) a substantial network of his own (and through his family) that he can draw on and (as noted) near universal name-recognition. None of the other major contenders comes close in that regard.
8. Hence, Romney’s entry – even if it is ridiculous – hurts the other potential establishment-acceptable candidates (like Chris Christie and Scott Walker) in this early phase of the contest much more than it hurts potential insurgent candidates like Ted Cruz, Rand Paul or Mike Huckabee.
9. An insurgent candidate by definition cannot be the front-runner until the insurgency has demonstrated very substantial successes in the field – and certainly not during the invisible primary phase.
10. Hence, if Romney’s entry is ridiculous, Jeb Bush is the front-runner.
UPDATE: Just to be clear, by “front-runner” I mean “shortest odds to win” not necessarily “more likely than not to win” much less “prohibitive favorite.” Saying “there is no front-runner” means that you think the odds are pretty well even across a group of candidates.
My syllogism above is therefore compatible with Daniel Larison’s proposition that Romney’s entry makes it somewhat more likely that an insurgent candidate will take the nomination – provided that you also recognize that there are multiple insurgent candidates (so you are comparing one candidate’s odds with the cumulative odds on any member of a group) and that, with or without Romney in the race, none of the insurgents have very good odds of winning the nomination.